While less recognized than their animal counterparts, many non-neuronal organisms, such as plants, bacteria, fungi and protists, also have the ability to make complex decisions in difficult environments. ~ Australian ethologist Chris Reid
Slime molds are early evolved eukaryotes that can live freely as single cells but aggregate to form multicellular reproductive structures. Their calumnious characterization refers to that part of their lives when they appear as gelatinous slime. Many slime molds spend little time in such a state, preferring a less-slick solitude.
Slime molds smartly forage, avoiding where they have been. As it moves, a slime mold leaves a chemical trail that lets it identify its own secretions. These trails serve as an external spatial memory.
Though they exhibit sophisticated behaviors, slime molds are beyond brainless. They lack nerve cells altogether.
Physarum polycephalum is an exemplary slime mold. It is one of the easiest of eukaryotic microbes to grow in culture. As such, P. polycephalum has been a favorite subject for study, and so is the best known. What is true about the intelligence of P. polycephalum doubtlessly applies to Physarum in general, and quite possibly slime molds altogether.
In its natural habitat, a Physarum spends its days in shady, cool, moist areas of the forest, foraging on logs and decaying leaves.
Physarum is a protist with a purpose. Amoeba-like, it spreads out in search of prey (bacteria and fungi), as well enjoying salads of decomposing vegetation. Physarum extends itself by oozing tendrils along the forest flower.
The problem is knowing which direction to grow. Physarum proceeds intelligently.
The foraging algorithm centres around a tendency to exploit environments in proportion to their reward experienced through past sampling. The algorithm is intermediate in computational complexity between simple, reactionary heuristics and calculation-intensive optimal performance algorithms, yet it has very good relative performance. ~ Chris Reid et al
Though their lifestyles are incomparable, the mental acumen of Physarum is equipotential to humans, even in making irrational choices.
This unicellular protist lacks a central nervous system and possesses no neurons, yet it has been demonstrated to solve convoluted labyrinth mazes, find shortest length networks and solve challenging optimization problems, anticipate periodic events, use its slime trail as an externalized spatial memory system to avoid revisiting areas it has already explored, and even construct transport networks that have similar efficiency to those designed by human engineers.
Slime mould cells also display similar decision-making constraints to the cognitive constraints observed in brains. P. polycephalum is vulnerable to making the same economically irrational decisions that can afflict humans.
Like humans, slime moulds are subject to speed-accuracy trade-offs when confronted with a difficult choice. Certain problem-solving processes, as well as their associated trade-offs and paradoxes, are spread wide on the phylogenetic tree. ~ Chris Reid et al
Slime molds have long-term memory. They learn patterns and anticipate periodic events. Further, slime molds pass their knowledge on to others as pillow talk.
Slime moulds exhibit transfer of learned behaviour during cell fusion. ~ French biologist Audrey Dussutour & biologist David Vogel
Slime molds demonstrate memory and problem-solving without having any identifiable physical structure that supports these abilities. Sublime slime shows that intelligence has an energetic foundation.